Welt, cording, piping... That thin rope like feature on all the cushions and trim you've been seeing. That's welt. Buying the cord in bulk is your safest bet. You're going to need a ton of it if you're getting into upholstery. Welt doesn't just serve as a decorative trim that hides your staples. It's stiff nature also helps to hold the shape of box cushions, and can provide striking contrasting detail to your furniture projects. It also comes in many different thicknesses, shown here in 32nds of an inch increments. Size is typically relative to your project or taste. There are no hard fast rules. Want to make your life a ton easier? Purchase a single and double welting foot for your most commonly used sizes. You can get these online for just about any type of machine, domestic or industrial. If you can't get a welting foot, a zipper foot will also make it easier for you to get closer to the rope. Speaking of rope, you can just use rope from the hardware store for small projects or to conserve space in your workshop, wherever that may be. These rolls can take up a lot of space, and if you don't use it regularly, makes more sense to buy it in smaller batches. You can get 100 feet at Home Depot for under $10.